ISANA, 36; 72-74 (2002)

Mouth opening in dolphins, as revealed by magnetic sensors (originally in Japanese)

Ropert-Coudert, Y., N. Liebsch, A. Kato, G. Bedford, M. Leroy, R.P. Wilson 2002

FULL TEXT IN ENGLISH (with the authorisation of the editor):

Measuring the feeding activity of free-ranging marine mammals, especially cetaceans, by non-invasive and non-lethal method has always been a challenge to researchers. In this regard, jaw movement detectors were tested on free-ranging Weddell seals, Leptonychotes weddelli, in tandem with other data-loggers, giving general indication on the preferential feeding zones exploited by the seals (Plötz et al. 2001). An improved system that allows researchers to detect the exact timing of prey capture, as well as to estimate the size of the prey ingested by measuring the angle of beak opening, was successfully applied on free-ranging penguins (Wilson et al. 2002). This latter system was tested in October 2001, on various captive marine animals (seals, sea-lion, turtles and dolphin) at the "Sea World Enterprises", a marine theme park situated on the Gold Coast, Queensland, Australia. The present note focuses on the use of the jaw detector system on a captive inshore Bottlenose dolphin, Tursiops truncatus aduncas, called "RB".

RB is a 12 years old, 120.5kg dolphin, trained through the process of positive reinforcement using hand cues to elicit responses. An ultra-sonic whistle is used as a bridging stimulus.

Jaw movements were detected as variations in the strength of the magnetic field between a magnet and a Hall sensor (GmBh, Germany). The Hall sensor is linked by a cable (0.8 mm in diameter) to a round, 35 x 15 mm, one channel IMASU data-logger (Driesen and Kern, GmBh, Germany, see details in Wilson et al. 2002). The logger was set to record the magnetic field at 24 Hz. Three square Rare Earth magnets (15 x 4 x 2 mm) were embedded in foam and cloth-backed TESA tape (Fig. 1), and were glued using cyanoacrylate glue (UHU Sekundenkleber, GmBh, Germany) on the upper jaw of the dolphin nearby the corner of the mouth. The Hall sensor was also embedded in foam and cloth-backed tape and was glued on the lower jaw of the dolphin, directly facing the magnet. Finally, the logger, embedded in a silicon handmade suction cup, was attached on the left lower cheek below the eye.

Attachment and retrieval of the logger were performed by the trainer. At the end of the experiment, the system was easily removed by pulling gently on the top of the suction cup or on the silicon, indicating that the system of attachment would not be suitable for long-term deployment. The feeding session was conducted while RB was swimming in a circular 331 987m2 (6 000 000litres) pool, 3 m deep. The whole feeding session was filmed using a JVC GR-DVL100 digital video camera in order to further relate the prey intake filmed by video camera to the variations in the magnetic field recorded by the logger. The feeding session lasted 6 min, during which RB swallowed 10 dead fish (mainly Yellowtail, Whiting, Herring, Mullet, and Goatfish) under the water.
The strength of the three embedded magnets was too great for the sensor, which was saturated every time the dolphin kept its mouth closed. "RB" opened its mouth at every surfacing event, either swallowing fish given as a reward or squeaking for rewards, rendering the analysis complex. However, using the video, the patterns of fish swallowing could be determined from the logger data (Fig. 2a). Swallowing started by a long, 0.66 ± 0.3sec and important, 2348.2 ± 134.3mV (amplitude of the change in the voltage) opening of the mouth, where fish were initially secured. This big opening was almost immediately followed by series of brief 0.32 ± 0.15sec and slightly smaller, 1846.3 ± 394.3mV jaw movements, which were performed probably in order to orientate the prey towards the back of the mouth. In two cases, the first opening of the mouth was smaller and shorter than the successive opening (example in Fig. 2b). This second larger drop may correspond either to another activity, i.e. squeaking or drinking, or to a second capture event, the fish being missed at the first opening.
Timing of prey intake and patterns of swallowing were, therefore, clearly determined. It is likely that analysis of the area, duration and/or amplitude of the first large voltage drop may help us determine the size of the prey, as it was demonstrated on penguins (Wilson et al. 2002). Unfortunately, the short duration of the feeding session and the absence of information on the morphometrics of the fish given to RB prevented us to investigate further the capacity of the logger to measure quantitatively the amount or size of the prey ingested.
Finally, a vocalization induced by the trainer (Fig. 3) was also recorded, corresponding to a single large drop in the voltage. The syncopated pattern of RB's squeaking ("eek - eek - eek") was identified in the data by several small undulation, regularly interspaced at the bottom of the voltage drop. This suggests that researchers could measure "quantitatively" the vocalizations produced underwater by free-ranging dolphins, leading to a better understanding of the way cetaceans communicate with each other. Future experiments should focus on increasing the sample size of prey taken but this implies that the method of attachment of the logger improves substantially…

Acknowledgements: The authors wish to thank all the staff at the Coolangatta Sea World for their help and friendship and for authorizing us conduct experiments in their facilities.

Plötz J, Bornemann H, Knust R, Schroder A, Bester M (2001) Foraging behaviour of Weddell seals, and its ecological implications. Polar Biology 24: 901-909

Wilson RP, Steinfurth A, Ropert-Coudert Y, Kato A, Kurita M (2002) Lip-reading in remote subjects: an attempt to quantify and separate ingestion, breathing and vocalisation in free-living animals using penguins as a model. Marine Biology 140:17-27.


NIPR Marine Biology Group